Ignoring clinical diagnoses like multiple personality disorder, it can be helpful to think of one human mind as multiple conscious entities all renting the same apartment: your brain.
When you start thinking this way you’ll see evidence everywhere. “I don’t know what came over me”. The experience of suddenly becoming aware that you are driving. Waking up every morning with no recollection of the last seven hours. Walking through a doorway and instantly forgetting why you’re there. These are all cases where one person in your head is not talking enough to the other people.
Luckily, all the people renting space in your mind all have a common landlord that must approve all actions: the pre-frontal cortex. If you have a lazy landlord, practice sustained metacognition.